The present invention relates generally to a vapor compression system including an accumulator sized to protect the system against over-pressurization when inactive.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. “Natural” refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide as a refrigerant to run transcritically, or partially above the critical point, under most conditions, including when inactive. Under transcritical operations, pressure within the system becomes a function of both temperature and density.
A vapor compression system usually operates under a wide range of operating conditions. External atmosphere conditions, including temperature, can affect the pressure of the system while inactive. The system components (compressor, condenser/gas cooler, expansion device, evaporator and refrigerant lines) are designed to withstand a maximum pressure, but exposure to higher pressures may result in damage to the components. For most systems, the pressure in the system when not operational is a direct function of the temperature that the system is exposed to. However, when this temperature is near or above the critical point of the refrigerant, an additional factor must be considered. For supercritical fluids, the pressure in the system is a function of both the temperature and density of the fluid. This is not typically a concern for most refrigerants because their critical points are near or above normal storage temperatures. For carbon dioxide (CO2) systems, however, this becomes an issue because the critical point is very low (88° F.).
A relief valve is typically incorporated into the system to protect the system and the components against over-pressurization. If pressure in the system approaches an over-pressurization point, the relief valve automatically opens to discharge refrigerant from the system and decrease the pressure to a safe range to protect the components from damage.
Vapor compression systems are typically designed to be stored at a certain maximum temperature, and the system components are designed to be able to withstand the maximum pressures associated with this temperature. The higher the storage temperature, the higher the design pressure usually needs to be. When the storage temperature is near or above the critical temperature of the refrigerant, the bulk density of the refrigerant is important in determining the system pressure, and therefore the design pressure. This is shown schematically in FIG. 1, which illustrates how the system pressure changes above the critical point for carbon dioxide as a function of both temperature and bulk density.
Prior vapor compression systems include an accumulator positioned between the evaporator and compressor that stores excess refrigerant. The accumulator is only sized to provide enough capacity for storing excess refrigerant during operation to prevent the excess refrigerant from entering the compressor. The accumulator can also be used to control the high pressure, and therefore the coefficient of performance, of the system during transcritical operation. However, the accumulator is not sized to determine a maximum pressure when the system is inactive or in storage.
Hence, there is a need in the art for a vapor compression system that includes an accumulator sized to prevent over-pressurization of the system while inactive, and a method for sizing such accumulator.